Myotonic dystrophy (DM) is the most common form of adult onset muscular dystrophy affecting 1 in 8500 people worldwide. It is dominantly inherited and affects multiple organ systems. Causative mutations are expanded tri-(CTG) and tetra-(CCTG) nucleotide repeats in transcribed but non-translated genomic regions. A major pathogenic mechanism of DM involves a toxic RNA gain-of-function caused by expression of RNA transcripts from the expanded alleles. The goal in this proposal is to determine the molecular mechanism by which RNA containing the expanded repeats causes progressive skeletal muscle dystrophy and cardiac arrhythmias, the predominant causes of mortality and morbidity. In the previous funding period we demonstrated that the pathogenic mechanism involves misregulation of alternative splicing and we identified target genes responsible for specific symptoms. We also found, that these targets are regulated antagonistically by two families of RNA binding proteins both identified previously based on CUG RNA binding activity. We will define the mechanism by which expanded CUG RNA induces pathogenesis with specific emphasis on the roles of individual members of these two protein families. First, while it is clear that CUG repeat RNA is pathogenic, the specific form of the RNA required for pathogenicity is unknown. We will use an established assay to define the sequence, protein binding, and structural features of RNA required for induction of splice-misregulation in trans. Proteins whose direct interactions with the RNA correlate with aberrant RNA processing will be identified. Second, genes whose mis-regulation contributes to myopathy and arrhythmias will be identified using novel biochemical and subtractive approaches. Third, stable cell lines inducibly expressing expanded CUG RNA will be used to characterize the distribution and metabolism of toxic RNA and the consequences on the expression of the RNA binding proteins that are relevant to cell toxicity. Fourth, we will develop versatile lines of transgenic mice using a Cre/LoxP strategy to induce high levels of expanded CUG RNA in specific tissues and in early development. These studies will provide cellular and mouse models to define altered regulatory pathways and the genes affected by this novel disease mechanism. [unreadable] [unreadable] [unreadable]